1. Technical Field
Embodiments of the present invention relate to a virtual image display apparatus which provides video formed by an image display element or the like to an observer, and in particular, to a virtual image display apparatus which is suitable for a head-mounted display mounted to the head of the observer.
2. Related Art
Various optical systems have been proposed as an optical system which is assembled in a virtual image display apparatus, such as a head-mounted display (hereinafter, referred to as an HMD) mounted to the head of an observer (see JP-A-2-297516, JP-A-7-134264, and JP-A-7-151993).
In regard to the virtual image display apparatus, such as an HMD, it is desirable to enlarge a viewing angle of video light and to reduce the weight of the apparatus. In particular, in order to improve the fit, it is important to reduce the thickness in a visual axis direction of the observer and to bring the center of gravity close to the observer.
If the field of view of the observer is completely covered and thus only video light is viewed, an external scene is not noticed by the observer, causing uneasiness for the observer. Furthermore, the external scene and video are displayed in an overlapping manner, and accordingly various new usages, such as virtual reality, are produced. For this reason, there is a need for a display which displays video light in an overlapping manner without obstructing the field of view of the external scene.
In order to improve the fit for the observer and to improve the appearance of the form, it is desirable to arrange a video display device across the face, instead of being disposed above the eyes.
In order to reduce the size of the optical system and to position the video display device separately from the position of the eyes of the observer not to obstruct the field of view, it is preferable to image display image light on the optical system once to form an intermediate image, and a relay optical system which enlarges the intermediate image to be displayed.
However, when forming an intermediate image using a relay optical system, if there is a scratch, dust, or stain on the surface of an optical member near the intermediate image, since the scratch or the like is viewed on an enlarged scale for the observer, there is a problem in that video quality is significantly degraded. Even when an intermediate image is formed inside an optical member, such as a prism, if there is a contaminant or air bubble in the material of the optical member or discoloring or the like occurs locally in the optical member due to aging degradation, the contaminant or the like is enlarged, causing degradation in display quality.
For example, JP-A-2-297516 proposes a relay optical system which uses a parallel planar light guide plate, in which a parabolic mirror is provided at an end surface thereof, and a projection lens, and forms an intermediate image inside the light guide plate. However, in the case of the optical system of JP-A-2-297516, since the projection lens is large in size and heavy, the projection lens cannot be provided across the face, and the whole HMD increases in size. Since the intermediate image overlaps the surface of the light guide plate, a scratch or stain is displayed on an enlarged scale.
JP-A-7-134264 proposes an optical system which uses a relay lens optical system and is reduced in weight. In the case of this optical system, since an intermediate image is formed in a space between lenses, a contaminant or a scratch is unlikely to be viewed. However, in this optical system, a large half mirror and a concave mirror are required in front of the eyes of the observer, and the appearance is significantly damaged. The number of projection lenses is large, and the full length increases. In order to observe an external scene, since a concave mirror should be a half mirror, and video light is reflected by the half mirror two times and is further transmitted through the half mirror, video becomes very dark.
In order to reduce the size of the optical system, it is effective to reduce the width of a light beam flux (light flux) passing through the optical system. If a sufficient light beam flux is not provided at the position of the pupil of the observer, the visual field of video is lacking, and some person cannot observe video depending on an individual difference in pupillary distance. Considering this point, in JP-A-7-151993, a relay optical system is configured, and a diffusion plate is placed at the position of the intermediate image to diffuse a light beam, and accordingly the light beam flux width in the pupil is expanded. However, since an image on the diffusion plate is enlarged by an eyepiece lens system, a contaminant or a scratch on the diffusion plate is also enlarged, and there is a concern that video quality is damaged.